The present invention relates to methods and apparatus for heat treating surfaces and particularly for heat treating metal surfaces by directing a high power laser beam to the surface.
Metals are heat treated in different ways for different purposes. For example, high carbon metal parts such as springs are made stronger by heating them above their critical temperature and then cooling them. A journal or a shaft may be hardened by a number of techniques so that it will wear better. One technique for low carbon metal parts such as shafts is to heat the shaft in an atmosphere of selected gas or liquid so that materials dissolve from the gas or liquid in the surface metal of the shaft. A hard surface is then produced by heating and a rapid quench. The depth of this hardening depends upon the temperature and time of exposure to the atmosphere. This is a conventional process and is called "case hardening". Three commonly used types of case hardening are carburizing, nitriding, and cyaniding. For example, a steel shaft is case hardened by heating the shaft in an atmosphere of CO.sub.2 to a temperature in the range of 1700.degree. F. and at this temperature exposing the shaft to the CO.sub.2 gas for a period of an hour or two and then quenching the shaft. Minute amounts of carbon are liberated on the surface of the hot metal and dissolve in the metal. Upon quenching, the carbon becomes part of the crystalline structure of the metal at the surface.
Case hardening of high carbon metals can also be accomplished by induction heating. An induction coil enclosing the metal piece to be case hardened induces an electromagnetic field and currents in the metal that flow just along the surface whereby the surface of the metal piece is preferentially heated. If the surface is thusly heated above the critical temperature and then the piece is quenched, the surface only of the metal piece becomes hardened. Clearly, the conventional techniques for case hardening, whether heating a metal piece in a selected gaseous atmosphere as in the carburizing process, or by selectively heating only the surface of the piece by induction are quite limited as to the shape and size of the pieces that can be case hardened. For example, it is difficult to harden only selected portions of the surface of a shaft using either the carburizing technique or the induction heating technique. Thus, by these conventional techniques, odd geometries or selected portions of a piece are most difficult to preferentially harden. Furthermore, the ability to control the depth of the case hardening whether using the carburizing technique or the induction heating technique is quite limited.
Recently, it has been proposed to selectively heat the surface of a metal piece by directing a high power laser beam to the surface over areas of the surface which are treated with a material selected, for example, to absorb the energy of the beam. The advantages of this technique are that selected areas of the surface of the metal piece can be coated with the material so that when the beam sweeps the surface of the metal piece, only those areas covered with the material will be heated. The scanning laser beam selectively heats the surface of the metal piece to a temperature above the critical temperature without raising the bulk temperature of the piece sufficient to cause any serious distortion or other affects of heating. The power density of the beam may be controlled to heat the surface of the metal piece above the critical temperature to a depth of from a few thousandths of an inch to about fifty thousandths of an inch. Thereafter, the quenching of the surface occurs by conduction of heat out of the surface into the base metal. Since the surface layer heated by the laser beam is shallow, the conduction quench rate is very fast.
A high power laser beam may be a solid or pencil laser beam that is an inch or so in diameter away from the focal point and a fraction of an inch in diameter at the focal point. The intensity distribution across the diameter of this beam, also called the intensity profile or intensity shape of the beam, depends very much on the type of laser. One common profile or beam shape for a pencil beam is a Gaussian shape whereas in some lasers the laser beam away from a focal point is annular or hollow and so in this case, the intensity profile is generally U-shaped. If the sweeping laser beam directed to the surface of a metal piece has a distinct Gaussian shape, it is quite clear that heating of the metal surface at the center of the beam will be a great deal more intense than heating along the edge of the beam. When such a beam is swept across the metal surface and the beam scans do not overlap, the surface is not heated uniformly and so the depth of hardening will not be uniform throughout the area of the metal surface scanned by the laser beam. On the other hand, if the repeated scans of the laser beam on the surface of the metal overlap, the portion of a given scan which is overlapped on the next scan of the beam will cool somewhat between the scans and so either will not be heated sufficiently to raise the temperature above the critical temperature or the reheating by the subsequent scan will anneal the metal at the surface where the repeated scans overlap and so negate the hardening affect at the overlapping areas. The result is that hardening of the scanned metal surface is not uniform in hardness nor in depth of hardening.
One type of high power laser is a flowing gas electron beam energized CO.sub.2 laser. A laser of this type is described in U.S. Pat. No. 3,702,973 which issued Nov. 14, 1972 and another is described in U.S. Pat. No. 3,808,553 issued Apr. 30, 1974. These lasers produce an annular or ring-shaped laser beam having a power on the order of 10-20 kilowatts and so these lasers are suitable for heating a surface of a metal piece to, for example, case harden the surface. The beam from these lasers is annular in shape and hence have a generally U-shaped as distinguished from a Gaussian intensity profile except at the focal point of the beam where the beam characteristics are that of a Fraunhofer diffraction pattern with a central core containing anywhere from 5 to 80% of the power of the beam and the remainder of the beam power being located in concentric Airy rings around the central core. The intensity distribution at locations of the beam other than the focal point depend upon the beam divergence angle and the annular ratio. However, in general, the intensity distribution or profile of this annular beam always contains rings around a central maximum as well as the possibility of a depression in the middle of the beam due to the near field annular characteristics of the beam. In all cases, the intensity distribution or shape of the beam depends on the type of laser oscillator that is used and on the location along the beam relative to the focal point. Most often, however, the intensity profile is not ideal for uniformly heating the surface of a metal piece to, for example, case harden the surface. U.S. Pat. No. 3,848,104 issued Nov. 12, 1974 describes apparatus for spacially oscillating an annular high power laser beam focused substantially to a spot whereby the cross-section dimensions of the spacially oscillated or "dithered" laser beam is substantially greater than the initial beam at the point of impingement on the surface to be treated and covers an area considerably greater than that which would be covered by the same beam were it not dithered. The surface to be treated is then swept by the "dithered" beam.
It is a principal object of the present invention to provide a technique for producing from an input annular laser beam a laser beam that is tailored to provide at the surface of a work piece an annular shape of desired cross sectional area and intensity profile independent of that of the input laser beam.
U.S. Pat. No. 4,017,708 issued Apr. 12, 1977 describes a method of heat treating a bore in a work piece that may utilize an axially positionable rotating conical mirror structure with a single continuous reflective conical surface to receive an annular output laser beam and direct it onto the inner surface of a bore in a work piece such as a cylinder sleeve. The conical mirror provides a generally circumferentially disposed heating zone or ring-shaped focal pattern by focusing and reflecting as a single operation the impinging annular laser beam on the surface to be processed to provide a heating zone of the desired width but having the same energy profile as the output laser beam.
A conventional conical mirror of the type disclosed in the aforementioned U.S. Pat. No. 4,017,708 is effective only to decrease the impinging laser beam width as a function of cone angle. Such a conventional conical mirror will project an annular laser beam of the desired width for only a narrow range of conditions and requires accurate beam alignment and good beam symmetry. Further, the energy profile or distribution of the output laser beam in this case is not amenable to modification. Thus, such an arrangement is not satisfactory, for example, where the input laser beam has an unsatisfactory and/or nonuniform radial energy distribution in either the focused or unfocused condition. Such an arrangement is further unsatisfactory, for example, for heat treating a valve seat where compensation of the energy distribution of the laser beam is necessary for the heat sink effect of the work piece and to prevent melting at the edges of the valve seat. Another application where the capability to modify the energy distribution of a laser beam is important is for heat treating the outer surface of shafts and the surface of a bore. In these cases, we have found that in many cases, it is desirable to have a higher beam intensity at the leading edge of the beam as it traverses the surface to be treated.